Synchronizing gas injections and time-resolved data acquisition for perturbation-enhanced APXPS experiments

An experimental approach is described in which well-defined perturbations of the gas feed into an Ambient Pressure X-ray Photoelectron Spectroscopy (APXPS) cell are fully synchronized with the time-resolved x-ray photoelectron spectroscopy data acquisition. These experiments unlock new possibilities for investigating the properties of materials and chemical reactions mediated by their surfaces, such as those in heterogeneous catalysis, surface science, and coating/deposition applications. Implementation of this approach, which is termed perturbation-enhanced APXPS, at the SPECIES beamline of MAX IV Laboratory is discussed along with several experimental examples including individual pulses of N-2 gas over a Au foil, a multi-pulse titration of oxygen vacancies in a pre-reduced TiO2 single crystal with O-2 gas, and a sequence of alternating precursor pulses for atomic layer deposition of TiO2 on a silicon wafer substrate.Peer reviewe

AP-XPS Study of Ethanol Adsorption on Rutile TiO2(110)

The photoactivity of rutile TiO2(110) renders its surfaces of particular interest for the study of surface reactions. In particular, rutile TiO2(110) surfaces are active for hydrogen production, both via the water splitting process and via ethanol degradation under ultraviolet illumination. The selective photocatalytic dehydrogenation of rutile TiO2(110) is not fully understood yet, and an important question in this context is how ethanol adsorbs onto the rutile TiO2(110) surface under ambient conditions. Here, we present the first in situ experimental study on the absorption of ethanol on rutile TiO2(110) at room temperature and near-ambient conditions. The surface sensitivity of synchrotron-based ambient pressure X-ray photoelectron spectroscopy allows for an in-depth analysis of the surface species (molecular ethanol and ethoxies) and their coverage as well as an estimation of the energy difference between the two species. Through modeling of the O 1s core level and comparison to experimental results we show that both molecular and dissociative adsorption of ethanol occurs. The difference in adsorption energy range calculated from modeling of the O 1s core level was 0.018-0.033 eV, with dissociative adsorption the most energetically favorable. The difference in adsorption energy is almost an order of magnitude lower than previous estimations from theoretical calculations. In addition, we show that at room temperature a multilayer is formed with increasing pressure of ethanol

Oxygen relocation during HfO2 ALD on InAs

Atomic layer deposition (ALD) is one of the backbones for today’s electronic device fabrication. A critical property of ALD is the layer-by-layer growth, which gives rise to the atomic-scale accuracy. However, the growth rate - or growth per cycle - can differ significantly depending on the type of system, molecules used, and several other experimental parameters. Typically, ALD growth rates are constant in subsequent ALD cycles, making ALD an outstanding deposition technique. However, contrary to this steady-state - when the ALD process can be entirely decoupled from the substrate on which the material is grown - the deposition’s early stage does not appear to follow the same kinetics, chemistry, and growth rate. Instead, it is to a large extent determined by the surface composition of the substrate. Here, we present evidence of oxygen relocation from the substrate-based oxide, either the thermal or native oxide of InAs, to the overlayer of HfO2 in the initial ALD phase. This phenomenon enables control of the thickness of the initial ALD layer by controlling the surface conditions of the substrate prior to ALD. On the other hand, we observe a complete removal of the native oxide from InAs already during the first ALD half-cycle, even if the thickness of the oxide layer exceeds one monolayer, together with a self-limiting thickness of the ALD layer of a maximum of one monolayer of HfO2. These observations not only highlight several limitations of the widely used ligand exchange model, but they also give promise for a better control of the industrially important self-cleaning effect of III-V semiconductors, which is crucial for future generation high-speed MOS

Time evolution of surface species during the ALD of high-k oxide on InAs

Understanding the reaction mechanisms involved during the early stage of atomic layer deposition (ALD) of HfO2 on InAs is a key requirement for improving interfaces in III-V semiconductor-based devices. InAs is an excellent candidate to outperform silicon regarding speed and power consumption, and combined with HfO2, it gives promise for a new generation of ultra-fast MOSFETs. However, an improved interface quality and in-depth understanding of the involved surface species are needed. Here, we use in situ and operando ambient pressure XPS to follow in real-time the reaction mechanisms which control the ALD chemistry. Besides the removal of all unwanted oxide from the III-V, the same oxygen atoms are found to form HfOx already from the first half-cycle. In contrast to the standard ALD model, no hydroxyl groups are needed on the InAs surface. Furthermore, we observe an insertion reaction forming unexpected surface species. The second ALD half-cycle allows the immediate removal of all organic species leaving behind a uniform HfO2 layer partially terminated by hydroxyl groups. We find that prolonged exposure times upon both half-cycles guarantee a sharp InAs/HfO2 interface. Such an improved interface is an important step towards fast and sustainable III-V semiconductor-based electronics

Role of Temperature, Pressure, and Surface Oxygen Migration in the Initial Atomic Layer Deposition of HfO2on Anatase TiO2(101)

The atomic layer deposition of HfO2on a TiO2(101) surface from tetrakis(dimethylamido)hafnium and water is investigated using a combination of in situ vacuum X-ray photoelectron spectroscopy (XPS) and time-resolved ambient pressure XPS. Precursor pressures and surface temperature are tuned as to map the space state of the deposition. In the initial stages of ALD, a reaction mechanism based on dissociative adsorption dominates over a classic ligand exchange mechanism, typically evoked when metal-amido complexes and water are used as the precursors for metal oxide ALD. Surface species, including a dimethyl ammonium ion and an imine, are identified. It is found that they can be formed only if the active role of the TiO2(101) surface is taken into consideration. The temperature of the surface enhances the formation of these species based on an insertion reaction of a hydrogen atom, which then assists the formation of more than the expected monolayer of HfO2. A HfO2overlayer is produced already during the first half-cycle, enabled by a reduction of the TiO2support. Dosing water at high pressure allows hydroxyl formation, which marks the transition toward a well-described ligand exchange reaction type. From the experiments performed, we find that the ALD of HfO2at room temperature, performed at high pressure, is mainly based on dissociation and that no side reaction occurs. These insights into the ALD reaction mechanism highlight how in situ studies can help understand how deposition parameters affect the growth of HfO2and how the ALD model for transition metal oxide formation from amido complexes and water can be extended

Synchronizing Gas Injections and Time-Resolved Data Acquisition for Perturbation-Enhanced APXPS Experiments

An experimental approach is described in which well-defined perturbations of the gas feed into an Ambient Pressure X-Ray Photoelectron Spectroscopy (APXPS) cell are fully synchronized with the time-resolved XPS data acquisition. These experiments unlock new possibilities for investigating the properties of materials and chemical reactions mediated by their surfaces, such as those in heterogeneous catalysis, surface science, and coating/deposition applications. Implementation of this approach, which is termed perturbation-enhanced APXPS, at the SPECIES beamline of MAX IV Laboratory is discussed along with several experimental examples including individual pulses of N2 gas over Au foil, a multi-pulse titration of oxygen vacancies in a pre-reduced TiO2 single crystal with O2 gas, and a sequence of alternating precursor pulses for Atomic Layer Deposition (ALD) of TiO2 on a silicon wafer substrate

Atomic Layer Deposition of Hafnium Oxide on InAs : Insight from Time-Resolved in Situ Studies

III-V semiconductors, such as InAs, with an ultrathin high-κ oxide layer have attracted a lot of interests in recent years as potential next-generation metal-oxide-semiconductor field-effect transistors, with increased speed and reduced power consumption. The deposition of the high-κ oxides is nowadays based on atomic layer deposition (ALD), which guarantees atomic precision and control over the dimensions. However, the chemistry and the reaction mechanism involved are still partially unknown. This study reports a detailed time-resolved analysis of the ALD of high-κ hafnium oxide (HfOx) on InAs(100). We use ambient pressure X-ray photoemission spectroscopy and monitor the surface chemistry during the first ALD half-cycle, i.e., during the deposition of the metalorganic precursor. The removal of In and As native oxides, the adsorption of the Hf-containing precursor molecule, and the formation of HfOx are investigated simultaneously and quantitatively. In particular, we find that the generally used ligand exchange model has to be extended to a two-step model to properly describe the first half-cycle in ALD, which is crucial for the whole process. The observed reactions lead to a complete removal of the native oxide and the formation of a full monolayer of HfOx already during the first ALD half-cycle, with an interface consisting of In-O bonds. We demonstrate that a sufficiently long duration of the first half-cycle is essential for obtaining a high-quality InAs/HfO2 interface

HIPPIE : a new platform for ambient-pressure X-ray photoelectron spectroscopy at the MAX IV Laboratory

HIPPIE is a soft X-ray beamline on the 3 GeV electron storage ring of the MAX IV Laboratory, equipped with a novel ambient-pressure X-ray photoelectron spectroscopy (APXPS) instrument. The endstation is dedicated to performing in situ and operando X-ray photoelectron spectroscopy experiments in the presence of a controlled gaseous atmosphere at pressures up to 30 mbar [1 mbar = 100 Pa] as well as under ultra-high-vacuum conditions. The photon energy range is 250 to 2200 eV in planar polarization and with photon fluxes >10(12) photons s(-1) (500 mA ring current) at a resolving power of greater than 10000 and up to a maximum of 32000. The endstation currently provides two sample environments: a catalysis cell and an electrochemical/liquid cell. The former allows APXPS measurements of solid samples in the presence of a gaseous atmosphere (with a mixture of up to eight gases and a vapour of a liquid) and simultaneous analysis of the inlet/outlet gas composition by online mass spectrometry. The latter is a more versatile setup primarily designed for APXPS at the solid-liquid (dip-and-pull setup) or liquid-gas (liquid microjet) interfaces under full electrochemical control, and it can also be used as an open port for ad hoc-designed non-standard APXPS experiments with different sample environments. The catalysis cell can be further equipped with an IR reflection-absorption spectrometer, allowing for simultaneous APXPS and IR spectroscopy of the samples. The endstation is set up to easily accommodate further sample environments